Electroless plating refers to the autocatalytic or chemical reduction of metal ions in an aqueous solution to a metal which is deposited on a substrate. Components of the electroless plating bath include an aqueous solution of metal ions, reducing agents, complexing agents, bath stabilizers and a catalytic agent that operates at a specific metal ion concentration and within specific temperature and pH ranges. The base substrate, upon which the metal is plated, is usually catalytic in nature. Thus, the preferred preparation yields a substrate having a catalyzed surface and once the substrate is introduced into the electroless solution, uniform deposition begins. Minute amounts of the metal to be deposited on the substrate, i.e., nickel, will further catalyze the reaction. After the original surfaces are coated with metal, the deposition is autocatalytic. Electroless deposition then continues, provided that the metal ion and the reducing agent are replenished and the proper pH of the bath is maintained.
In electroless plating, metal ions are reduced to metal by the action of chemical reducing agents. The reducing agents are oxidized in the process. The catalyst may be the substrate or metallic surface on the substrate, as described above, which allows the reduction-oxidation reactions to occur with the ultimate deposition of metal on the substrate.
The metal ion and reducer concentrations must be monitored and controlled closely in order to maintain proper ratios and to maintain the overall chemical balance within the plating bath. The electroless plating deposition rate is controlled by selecting the proper temperature, pH and metal ion/reducer concentrations. Complexing agents may be used as catalyst inhibitors to reduce the potential for spontaneous decomposition of the electroless bath.
The chemical reducing agent most commonly used in electroless plating is sodium hypophosphite. Others include sodium borohydride, dimethylamine borane, N-diethylamine borane and hydrazine. The electroless nickel baths are generally of four types: (1) alkaline nickel phosphorus; (2) acid nickel phosphorus; (3) alkaline nickel boron; and (4) acid nickel boron.
The alkaline nickel phosphorus baths plate at relatively low temperatures which makes them suitable for plating on plastics, especially plastics used in the electronics industry. A typical composition of an alkaline nickel phosphorus bath is: nickel sulfate -30 g/l, sodium hypophosphite -30 g/l, sodium pyrophosphate -60 g/l, triethanol amine 100 ml/l, pH 10.0 and operated at 30-35 degrees C.
Acid nickel phosphorus baths are commonly used for engineering applications since the deposits of nickel-phosphorus are quite hard, with excellent wear and corrosion resistance. The pH of the bath is the principal factor affecting the phosphorus content of the deposit. In general, the higher the pH, the lower the phosphorus content of the deposit. A typical acid nickel bath is: nickel sulfate -28 g/l, sodium acetate -17 g/l, sodium hypophosphite -24 g/l, lead acetate -0.0015 g/l, pH 4.6 and operated at 82 to 88 degrees C. There are many potential and actual formulations for hypophosphite, borane and hydrazine reducing baths. However, in all cases the nickel ion is reduced to nickel and the reducing agent is mostly oxidized but, to a lesser extent, may be reduced to become part of the nickel deposit.
In practice, several reducing agents may be used to form composites of nickel with diamonds, silicon carbide and polytetrafluoroethylene for special uses; and several polyalloys are produced including nickel-cobalt-phosphorous, nickel-iron-phosphorus, nickel-rhenium-phosphorus, nickel-molybdenum-boron, nickel-tungsten-boron and others.
There are also many formulations that may be used as the electroless nickel baths. However, in any such bath, an oxidation-reduction reaction occurs which results in oxidation products and metallic nickel. The pH decreases with removal of metal cations leaving anions of the nickel salt or complexing agent and the oxidation products of the reducing agents; i.e., hypophosphite to phosphite. The nickel ion and the reducing agent concentrations decrease with deposition. It is essential that the complexing agents, bath stabilizers and other additives remain in the bath at acceptable concentrations as the nickel is being deposited to prevent spontaneous decomposition of the bath and to minimize the number of chemicals that must be monitored and controlled.
Currently used electroless nickel baths have a limited life. The pH of the bath must be constantly adjusted with either an acid, usually sulfuric acid, and a base, usually sodium hydroxide. The combination of hypophosphite oxidation producing a phosphite and the reduction of nickel ions to metallic nickel usually results in excess acidity. This requires the addition of sodium hydroxide to obtain the required pH.
Objects of the instant invention are (1) to provide a process for controlling pH of the bath with a substantial reduction in the need to add sulfuric acid or caustic soda; (2) to remove the oxidized reducing agent efficiently so that the other components of the bath (reducing agent, metal ions, complexing agents and bath stabilizer) can be used for several cycles; and (3) to facilitate operation of the bath by providing a process wherein metal ion concentration and pH are controlled effectively and efficiently with a minimum of effort.